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Hatsujiro Hashimoto, widely known as “Hashi”, died on 15 April at the age of 95. Even by the exalted standards set by so many of his compatriots, the achievements of his 60-year active career in electron microscopy were extraordinary. Not only was he a pioneer in several crucial developments and applications but he played a leading role in encouraging electron microscopy at international level. In addition to his scientific reputation, he brought to this challenge his unique blend of pertinacity, diplomacy and artistic skill.

Stimulated by Ondrej Krivanek's contributions, the complex interaction between research and innovations in the instrumentation for electron microscopy is discussed. Specific attention is given to aberration correction and to spectroscopy in both the valence region and at the energies of localised phonons or bond vibrations. Current thinking about projection imaging and dielectric excitation theory may be challenged. More significantly in the new fields of investigation opened up to them, electron microscopists will need to build closer relations, particularly with the photonics and scanning tunnelling microscopy communities. Further improvements in instrumentation could usefully be directed towards imaging and spectroscopy at higher scattering angles as well as the incorporation of other facilities such as photon stimulation and secondary electron imaging.

Historical developments in electron microscopy where understanding lies at the interface between the particle and wave pictures of the electron are reviewed. Electron optics has remained mainly particle-based but imaging is now substantially dependent on wave methods which are now pushing it to new heights. Spectroscopy in the low loss region has used Fermi's classical electron model which can also describe the main effects of Johnson noise. It is confirmed that the RMS Johnson noise scattering of an aloof beam electron is proportional to its wavelength and to the square root of the temperature. If the conductivity σ is high, it is independent of σ. In the performance of magnetic spectrometers, the angular deflections due to the walls may be less significant than that generated in the slits. The angular deflections due to the many bremmstrahlung events in the spectrometer seem to be negligible but the influence of Johnson noise on them is beyond the scope of the classical Fermi theory and will need to be addressed by the quantum mechanical spontaneous wave theory.

Scanning tunneling microscopy (STM) images have been obtained in air of Berkovich indentations into Si(100), load range 2 to 25 mN, and into GaAs(100) at loads of 7 and 20 mN. The sides of the Si indentations appeared to be bowed inwards in contrast with the GaAs sides which appeared to be flat. Studies of hardness values were made using STM data and load-versus-depth curves from a nano-indenter. Tunneling spectroscopy results from both the Si indentation region and the undamaged surface indicate amorphisation of the deformed material.

GaAs–AlAs superlattices with periodicities of up to 200 nm were cleaved to expose (110) faces. These surfaces were studied using high resolution scanning electron microscopy where the secondary electron images reveal contrast between the GaAs and AlAs layers and also indicate that the AlAs layers are topographically raised with respect to the GaAs. Atomic force microscopy of the same samples shows that the AlAs ridges can extend as far as 55 nm beyond the GaAs surface. Further secondary electron images of cross-sections of plastically deformed GaAs–AlAs superlattices reveal that the AlAs layers can undergo severe deformation whereas the GaAs layers fracture more readily. It is proposed that the AlAs protrusions on the (110) cleavage surface are due to the fracture behaviour of the superlattice as well as oxidation effects.

Presented is a high-resolution scanning electron microscopy study of the mechanical behaviour of GaAs/AlAs superlattices that have been deformed through microindentation. Transverse cleavage through the indentation sites reveals the pattern of the deformed multilayers, showing in particular debonding between the GaAs and AlAs layers, regions of compressive strains of up to 30% and substantial slip on the {111} planes.

The understanding and where possible the minimisation of decoherence mechanisms in electron microscopy were first studied in plasmon loss, diffraction contrast images but are of even more acute relevance in high resolution TEM phase contrast imaging and electron holography. With the development of phase retrieval techniques they merit further attention particularly when their effect cannot be eliminated by currently available energy filters. The roles of electronic excitation, thermal diffuse scattering, transition radiation and bremsstrahlung are examined here not only in the specimen but also in the electron optical column. Terahertz-range aloof beam electronic excitation appears to account satisfactorily for recent observations of decoherence in electron holography. An apparent low frequency divergence can emerge for the calculated classical bremsstrahlung event probability but can be ignored for photon wavelengths exceeding the required coherence distance or path lengths in the equipment. Most bremsstrahlung event probabilities are negligibly important except possibly in large-angle bending magnets or mandolin systems. A more reliable procedure for subtracting thermal diffuse scattering from diffraction pattern intensities is proposed.

Despite the sub-angstrom resolution capability of modern transmission electron microscopes, elucidation of the structure of amorphous materials by direct-imaging methods is still challenged by problems of projection and data presentation identified over 20 years ago. Classical diffraction analysis, leading to the radial distribution function not only retains its essential importance but has been extended to deal with small scattering volumes in inhomogeneous samples by the use of focused electron probe illumination but retaining orientation averaging. Fluctuation microscopy provides a significant advance in the detection and statistical representation of medium-scale spatial variations in amorphous structure. Issues of scattering coherence are crucial in both these approaches. Further opportunities for use of focused probes include the study of angular correlations in nanodiffraction patterns as well as fine structure in energy loss spectra. Finally, there may be a return to the direct imaging approach th...

Experimental observations of static bending and flexural vibration modes in nanotubes are reviewed in the light of recent statements, linked to density functional theory simulations, that such modes do not exist. The evidence for flexural modes at wavelengths long compared with the diameter of the nanotube is overwhelming.

Image contrast in transmission electron microscopy is often three or more times less than theory predicts. Recent off-axis holography suggests that the origin of this Stobbs factor may lie in thermal diffuse scattering. In the light of this result, possibilities for further observations by TEM, STEM, CBED and holography are suggested as well as frozen phonon computations that include flexural modes.

Recent advances in instrumentation for electron optics and spectroscopy have prompted exploration of ultra-low excitations such as phonons, bond vibrations and Johnson noise. These can be excited not just with fast electrons but also thermally or by other external sources of radiation. The near-field theory of electron energy loss and gain provides a convenient platform for analysing these processes. Possibilities for selected phonon mapping and imaging are discussed. Effects should certainly be observable in atomic resolution structure imaging but diffraction contrast imaging could perhaps be more informative. Additional exciting prospects to be explored include the transition from phonon excitation to single atom recoil and the boosting of energy loss and gain signals with tuned laser illumination.

Zbigniew Basinski, widely known as Bas, was exposed early in life to turbulent events in Poland, Russia and Palestine. He studied chemistry at Oxford, then performed research in metal physics with Jack Christian (F.R.S. 1975). There he met Sylvia Pugh, his wife and lifelong partner, in an enduring and passionate study of crystal plasticity pursued largely in Canada. Over most of the period since the dislocation concept transformed the problem of explaining crystal weakness into one of explaining crystal strength, Basinski's experimental investigations of the obstacles to dislocation motion surpassed in quality and variety those of any other worker.

The work function ϕ, the bandgap Eg, the threshold energy level Et, for the inelastic scattering of excited electrons and the threshold energy transfer Ed for the onset of structural ionisation damage are clearly of major significance in various actively developing forms of hot carrier imaging. Two exciting examples here are the ability to image small and dynamic local changes in work function by PEEM and as well as mapping the variations in electronic structure between p and n-type regions of a semiconductor by SE imaging in the SEM. More recently still there have been indications that the SE signal in the ESEM might be sensitive to local changes in bandgap and suggestions that it might be even possible to image spatial variations in pH. It is increasingly clear that if these attractive opportunities are to be efficiently explored and developed, systematic and preferably quantitative observations are needed. Such work requires specimens whose atomic and electronic structure is either fully known beforehand or can be deduced from other signals available in a situation where the physical processes in the microscope are well understood.

Valence losses in electron microscopy can be conveniently computed using classical dielectric excitation theory. In many practical situations, the presence of complex structures gives rise to the possibility of coupling between interface modes, surface modes, and edge modes. In the nonrelativistic case, the losses in such geometries can be calculated by taking a distribution of surface and interface points, each of which is associated with an interface charge. These charges interact self-consistently with each other as well as with the charge of the incident electron. Here, we have applied this boundary charge method to provide scanning transmission electron microscopy energy loss spectra near-edged structures such as truncated slabs and junctions formed by several media. We find the dominant modes associated with such systems and study their contribution to the characteristic energy loss functions. The significance of these excitations compared with the planar interface peaks is discussed in terms of the sample geometry with illustrations for the cases of an MgO cube and an Si/SiO2 thin-film interface. @S0163-1829~99!00939-X#

Electron loss and electron-induced photon-emission probabilities are calculated near arbitrarily shaped dielectrics described by frequency-dependent response functions. The exact solution of Maxwell's equations is reduced to self-consistent equations involving integrals over the interfaces. The particular case of axially symmetric interfaces of arbitrary shape is discussed in detail. Photon-emission probabilities are shown to be of the same order of magnitude as loss probabilities in some cases, suggesting the possibility of measuring electron-induced radiation as a new microscopy technique.

Various imaging modes in both STEM and SEM which are useful for characterising small particles are described with direct evidence that some correction of the direct image signal is often necessary because of additional scattering effects. Specific examples chosen are the effect of substrate scattering on annular dark field images, the influence of elastic or quasi-elastic scattering on energy-selected transmission imaging and the role of local variations in backscattering and stopping power in secondary electron image contrast.

The partial wave phase shift formalism of atomic scattering is applied to compute exit wave functions for isolated Au and Si atoms under both plane wave and focused probe illumination. Connections between the far field and near field (exit) waves are clarified. This approach treats the Coulomb singularity properly though at 100 keV large numbers of phase shifts are required. In principle any form of incident wave can be handled so it may provide a means for testing traditional scattering theories used in electron microscopy. By applying the analysis to an atom embedded in a constant potential rather than free space, exit spheres of radius half the interatomic spacing can be used.

Dielectric response functions for stishovite, coesite, α-quartz and silacalite, obtained from electron energy loss spectra are studied and compared. The data for stishovite conforms well to simple dielectric theory for an insulator. However the behaviour of the other materials, including the relative insensitivity of the plasmon peak to density, is less well explained either on this basis or using more elaborate models based on Maxwell Garnett or Clausius-Mossotti effective medium theory. Possible improvements to effective medium theory are discussed.

Although dopant contrast in the scanning electron microscope has been known for a long time, its quantification is still a matter of debate mainly due to the lack of understanding of the contrast mechanism. Here we show that dopant contrast can be usefully increased at low extraction voltages. The effect may be related to the different angular and energy distribution of secondary electrons emitted from the p and n regions and for quantitative work should be studied over the full range of extraction potential.

High-angle annular dark field (HAADF) imaging in a dedicated scanning transmission electron microscope (STEM) has been applied to the study of imperfect crystals. Firstly, a study of B-doped layers in Si has revealed significantly stronger contrast and of opposite sign relative to simple atomic number contrast ( Z -contrast) predictions. It is shown that misfitting substitutional B atoms act as point defect sites in a Si matrix which enhance scattering to high angles via a static Debye-Waller effect. Multi-beam Bloch-wave theory has been used to quantitatively predict experimental contrast levels. Secondly, HAADF-STEM imaging of inclined dislocation segments revealed a number of novel contrast effects which depend on the specific position of the dislocation in the foil. Unlike conventional diffraction contrast from dislocations, HAADF dislocation contrast is neither similar nor complementary at the entrant and exit surfaces of the specimen. A qualitative Bloch-wave scattering description has been developed consistently to describe the dislocation contrast features.

Secondary electrons (SE) have been detected in coincidence with different energy-selected, primary loss events in the HB501 STEM. Results are presented for a number of materials - amorphous carbon, diamond, silicon carbide and magnesium oxide. In most cases transmission geometry was employed so that surface excitations, bulk valence excitations and core excitations are all involved and produce SE with different degrees of efficiency. Aloof beam experiments carried out on diamond and on magnesium oxide smoke cubes involve only surface valence excitations, however, and these produce SE with relatively high efficiency. Time of flight data are readily available from the coincidence spectra. Using computations for SE trajectories in the objective lens field, this information can be used to derive SE energy spectra. The interpretation of these experiments is discussed.

An outline is given of the extension of the classical theory of dielectric excitation to deal with the spatially localized valence electron spectroscopy of inhomogeneous media. The valence spectroscopy technique has been applied to mixed oxides of Hi and Al. The results have then been interpreted in terms of this extended theory. Significant variations in the position of the main collective loss peak were noted at different points on the samples and are tentatively ascribed to the presence of internal voids or pores rather than to variations in composition. Valence loss spectroscopy may thus provide a means of assessing local pore volume in mixed oxide catalysts.

High-angle annular dark-field scanning-transmission electron microscopic imaging of inclined dislocation segments revealed a number of characteristic contrast effects that have been shown to depend on the specific position of the dislocation in the foil. The dislocation contrast is initially very dark at the entrant foil surface due to stress relaxation-induced scattering of the incident electrons. Below the entrant surface of the foil, the dislocation exhibits oscillatory contrast which disappears for large incident beam divergence angles. Ultimately, the dislocation contrast remains bright beyond a certain depth in the crystal towards the exit foil surface. A qualitative Bloch-wave scattering description has been formulated to describe consistently the observed contrast features.

The Microstructural Physics group at the Cavendish Laboratory is actively involved in a considerable number of research projects which cover a broad range of materials science. In this paper, we describe briefly several such projects, with particular emphasis given to the application of parallel-detection electron energy loss spectroscopy (PEELS) on a scanning transmission electron microscope (STEM) to the analysis of materials such as stainless steels, catalysts, and high temperature superconductors. In addition, we describe a number of related projects that are currently being carried out in the group, particularly those which utilise and develop novel STEM imaging and analytical techniques. © 1993 Wiley-Liss, Inc.

The phase contrast imaging of surface steps in reflection electron microscopy is analysed. It is shown that the typical images are seriously affected by incoherence effects and that this arises, not from the strong inelastic scattering which is often present, but from the effective angular spread in the illumination which is frequently determined by the width of the Bragg reflection.

In the context of electron microscopists' changing attitudes to charging effects, some basic aspects of these phenomenona are surveyed. Methods of mapping internal charge distributions such as doping levels in semiconductors, trap distributions, or internal electric fields in insulators are discussed.

A variety of ways is described in which photons can be used not only for ultrafast electron microscopy but also to enormously widen the energy range of spatially-resolved electron spectroscopy. Periodic chains of femtosecond laser pulses are a particularly important and accurately timed source for single-shot imaging and diffraction as well as for several forms of pump-probe microscopy at even higher spatial resolution and sub-picosecond timing. Many exciting new fields are opened up for study by these developments. Ultrafast, single shot diffraction with intense pulses of X-rays supplemented by phase retrieval techniques may eventually offer a challenging alternative and purely photon-based route to dynamic imaging at high spatial resolution.

We discuss the interpretation of valence EELS data from composite media containing defects, clusters or precipates which may be difficult to resolve individually because of their small size or because of overlap effects. In some cases a well-defined collective loss (plasmon) peak is observed the position of which shifts slightly with changing defect concentration. In other cases discussed here the most noticeable effect is the appearance of new peaks in the spectrum which can be identified with interface as well as bulk excitations. Effective medium theories, although widely used to interpret optical absorption data from such samples, are only partially successful in explaining the energy loss spectra observed and do not account for the bulk loss peak from the minority phase. A simple dielectric excitation model where an appropriate average is made over the trajectories of a classical particle in the composite medium appears to offer a much better explanation and could be a useful way of characterising the fine-scale structure of composite samples. The connection between this trajectory approach and effective medium theory is discussed.

It is shown that valence loss spectroscopy can be successfully used to characterise the changes which occur in zeolite catalysts on dealumination and provide some evidence regarding the nature of the dealuminated state. The possible presence of water within the structure does not appear to affect the analysis. It is also shown that the spectra obtained can be fitted successfully using a Lorentz oscillator model and the changes in the strength and energy of the oscillators used could lead to a greater understanding of the effect of dealumination on the electronic structure.

In near coincidence with the event whose 75th anniversary is celebrated in this volume, the bold concept of crystal dislocations was put forward independently by Taylor,1 Orowan2 and Polanyi.3 This ultimate turning point in crystal plasticity spun the subject on its head by transforming the central ...

Recent applications of scanned probe microscopy are defined often going hand in hand with improved spectroscopy. These methods can provide information about dynamic response extending down towards the femotsecond time scale for resettable, repeatable phenomena and may thus help to fill the gap in the microscopist's coverage of material behavior. Although the use of photon pulses is best developed, ultra-short electron pulses are now available. Various options are therefore to hand in either transmission electron microscopy or scanning transmission electron microscopy operation for combining the spatial resolution of electrons with the spectral selectivity and precision of photons. With purely photon pump-probe operation, good spatial resolution can perhaps be obtained using the tip field-enhancement effect in scanning probe microscopy (SPM) and has also recently been impressively demonstrated in two-photon photo-electron microscopy (PEEM) operation.

The key contribution of electron microscopy methods to condensed matter spectroscopy is undoubtedly spatial resolution. So far this has mainly been manifest through electron energy loss spectroscopy in the 1-eV to 10-keV energy range and has not seriously challenged the dominance of optical, X-ray, and neutron spectroscopy methods over most of the vast field at lower energies. At frequencies up to a few megahertz, corresponding to energies of a few nanoelectron volts and below, direct excitation by pulsed electron beams or electric fields has proved effective. Prospects are discussed for extending spatially resolved spectroscopy to the intermediate energy region, mainly by combining the advantages of electrons with those of photons.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

The exact solution of Maxwell’s equations in the presence of arbitrarily shaped dielectrics is expressed in terms of surface-integral equations evaluated at the interfaces. The electromagnetic field induced by the passage of an external electron is then calculated in terms of self-consistently obtained boundary charges and currents. This procedure is shown to be suitable for the simulation of electron energy loss spectra when the materials under consideration are described by local frequency-dependent response functions. The particular cases of translationally invariant interfaces and axially symmetric interfaces are discussed in detail. The versatility of this method is emphasized by examples of energy loss spectra for electrons passing near metallic and dielectric wedges, coupled cylinders, spheres, and tori, and other complex geometries, where retardation aspects and Cherenkov losses can sometimes be significant., One of the authors (F.J.G.A.) wants to acknowledge help and support from the Spanish Ministerio de Ciencia y Tecnologia and the Basque Government.

Recent experiments prompt rethinking of the basics of elastic and inelastic electron scattering in electron microscopy. Standard approximations of elastic scattering largely based Bragg's law seem less clearly relevant when individual columns or even single atoms are probed. Phase shift analysis of atomic scattering can provide some checks and insights. The dielectric theory of aloof beam interactions is severely tested by observations of nanoparticle recoil but may be capable of explaining de-coherence effects induced by thermal fluctuations.

Oxidation and rapid reduction of supported organometallic precursors on silica yield large metallic particles (5 to 20 nm) that exhibit in transmission electron microscopy (TEM) a typical difference of contrast between the center and the borders. Z-contrast high angle annular dark field (HAADF) imaging in the scanning transmission electron microscope (STEM) has been successfully used to show that these particles are hollow in the case of Pd. PdMn particles have also been prepared and it is shown that they exhibit surface cavities instead of internal cavities.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Field-emission scanning electron microscopy (FE-SEM) has been used to study several semiconductor multilayer heterostructures. Compositional superlattices based on Ge x Si 1−x /Si and Al x Ga 1−x As/GaAs have been studied in both cross-sectional and oblique plan-views after indentation. Secondary and backscattered electron images reveal strong atomic number contrast which is primarily structural in origin. Secondly, for the first time, heterostructures containing n and p doping have been directly imaged at low voltages (0.5-1 kV) including: (i) Si- and Be-doped GaAs layers and (ii) B- and As-doped Si layers. Secondary electron images reveal strong contrast at doping concentrations as low as 10 17 cm −3 . The results have been interpreted in terms of energy band-bending effects between n- and p-doped layers

At relatively large scattering angles, Z -dependent effects occur in both the atomic (elastic or quasi-elastic) electron scattering and in the inelastic electron-electron scattering and can thus exhibit chemical contrast at high spatial resolution. Results are shown for secondary electron imaging of semiconductor multilayers in both bulk specimens and in transmission specimens. Variations in electron-electron scattering or stopping power can contribute to ADF images in the STEM and it may be useful to apply energy filtering as well as energy selection in their collection.